The invention relates to a method for controlling catalytic converter heat losses during coasting shutoff in an internal combustion engine having fully variable valve control.
Coasting shutoff is a known procedure used in some automobiles whereby, when an automobile is coasting (that is, rolling from its own momentum with the gas pedal not depressed), fuel is no longer injected into the engine. Thus, although the engine continues to turn and the pistons continue to pump, no fuel is injected or burned by the engine.
Because the engine is still turning during coasting shutoff, fresh air is drawn into the engine cylinders by the pumping of the pistons and pushed out again through the exhaust system. Thus, the pumping function of the engine causes fresh air to be passed continuously through the catalytic converter. This causes the catalytic converter to cool off, thus diminishing its effectiveness.
In conventional throttle-controlled spark ignition engines, the undesired catalytic converter cooling effects can be minimized by closing the throttle valve during coasting shutoff, and thereby preventing fresh air from being drawn into the engine.
However, engines equipped with fully variable valve stroke control typically do not include a throttle. Instead, the valve stroke of the Intake valves is adjusted continuously from zero stroke (no intake) to the maximum possible stroke using suitable valve positioners, for example, hydraulic actuators, electromagnetic actuators, or mechanical variable valve actuators, such as an adjustable cam shaft. Therefore, it is not possible to control catalytic converter heat losses during coasting shutoff by closing the throttle valve.
The invention relates to a method for controlling catalytic converter heat losses during coasting shutoff for an internal combustion engine having a valve, a valve positioner having an adjustable stroke set point, and a catalytic converter characterized in that the stroke set point is fixed as a function of a temperature of the catalytic converter.
In an exemplary embodiment of the invention, the stroke set point is fixed equal to or less than a first predetermined set point value when the temperature of the catalytic converter is less than a first predetermined temperature. In another exemplary embodiment, the stroke set point is fixed equal to or greater than a second predetermined set point value when the temperature of the catalytic converter is greater than a second predetermined temperature. An embodiment of the invention may, for example, fix the stroke set point to the first predetermined set point value when the temperature of the catalytic converter is less than a first predetermined temperature and fix the stroke set point to a second predetermined stroke set point value when the temperature of the catalytic converter is greater than a second predetermined temperature. The first predetermined set point value may, for example, be zero, corresponding to a closed valve. The second predetermined set point value may, for example, correspond to a full valve stroke.
The first predetermined temperature may be determined as a function of a minimum converter operating temperature. The second predetermined temperature may be determined as a function of a maximum permissible converter operating temperature. In order to prevent rapid switching of the stroke set points between the two predetermined stroke set point values, the second predetermined temperature is greater than the first predetermined temperature. The temperature of the catalytic converter can be calculated, for example, using a catalytic converter temperature model. The invention may function by fixing the stroke set point value of an inlet valve, but could also function by fixing the valve stroke set point value of an exhaust valve of the internal combustion engine.